ESD is a phenomenon of electrostatic motion along a nonconductive surface. Semiconductor devices and circuits in ICs may be damaged by ESD. For example, a charge carrying object, such as a human body walking on a carpet, a machine for packaging ICs or an apparatus for testing ICs, may discharge static electricity to the IC chip when they are in contact with each other. Thus, the IC chip is apt to be damaged or broken by the transient power of ESD.
Meanwhile, ESD is a transient process of high energy transformation from external to internal of an integrated circuit (IC) when the IC is floated. Hundreds or even thousands of volts are transferred during ESD. Such high voltage transformation is a common failure mechanism that is seen on integrated circuit devices. Due to the ever increasing shrinkage in transistor geometries, prevention of ESD damage will become increasingly important.
Currently, a LDNMOS device is an electronic device widely applied in the power management. When the internal device is a high voltage device such as a large-sized output driver, the output driver is coupled to a control circuit, and a gate-grounded LDNMOS device is used as an ESD protection device. The current LDNMOS ESD devices for high voltage process are almost manufactured by means of shortening the dimension of isolation structure, such as FOX (Field Oxide), so as to gain a lower breakdown voltage. However, this kind of shortening the dimension of the FOX is apt or prone to result in the problem of failure of the reliability.
Accordingly, it is preferable to design an LDNMOS device which can protect the internal device from ESD more completely.